Automatic transmissions for automotive vehicles have used mechanical, hydraulic, electrical or hybrid controls responsive to mechanical, hydraulic or electrical input signals. The most common type for automobiles and light trucks uses a hydraulic control system for determining optimum gear ratio with a manually operated, mechanically positioned range selector. The range selector usually specifically demands park, neutral, or reverse operation or a forward range comprising a few different gear ratios selectable by the hydraulic control dependant on demand and speed variables. The advent of microprocessor controls on automotive vehicles and the ever increasing reliability and cost effectiveness of such controls has brought proposals for extending electronic control to transmissions. An incentive for adopting these controls is the versatility of the controls and the relative ease of incorporating inputs other than those necessary for determining the proper gear ratio. Another incentive is to eliminate the traditional mechanical connection between the driver's station and the transmission.
U.S. Pat. No. 4,583,171 discloses a transmission control device having a microcomputer having vehicle load and speed for inputs as well as operator selection switches for desired range. The output of the microcomputer controls two actuators, a buzzer and some indicating lights for signifying the operator chosen range. The first actuator is for range selection and comprises a hydraulic servomechanism combined with an electromagnetic valve, or an electrically driven servomechanism combined with an electric motor. The computer decides on the basis of vehicle speed and the throttle angle whether the selected range is allowable. For example, to allow a shift from the park range to the reverse range, the vehicle speed must be near zero. The second actuator is operated to make the actual gear ratio shift as calculated by the microcomputer from the speed and load data.
U.S. Pat. No. 3,937,105 discloses a transmission control comprising a hard-wired logic circuit responding to the key switch state, speed, throttle position and range selection inputs for controlling a transmission having solenoid operated hydraulic valves for range and gear ratio control. Information regarding range is utilized only by the logic circuit; only specific valve operating commands for individual gear ratios etc. are presented to the transmission. The transmission includes a park mechanism. When the key switch is turned off and the vehicle speed is below some value (5 mph), the transmission is automatically shifted into park mode.